Power supplies in numerous portable products typically take up a substantial portion of the weight and volume of the portable product as a whole. The trend towards less voluminous and lighter power supplies is ever so evident in the communications industry. Thus, any further development that would decrease the overall weight and size of the power supply while maintaining (or increasing) the same charge capacity and life span of present packaged cell batteries would provide a distinct advantage. Typical batteries, such as nickel cadmium batteries, have at least one Ni-Cd cell. Each cell requires a metal casing enclosing a positive and a negative electrode, electrolyte, metal current collectors, and insulators. The term battery is occasionally used to identify a cell. However, it is more correctly used to identify a group of individual cells assembled in a pack with terminals (for electrical contact) and proper insulation such as a plastic housing for the battery pack containment. All this packaging results in an inefficient use of energy per unit volume of power source.
Today, in order to achieve the appropriate voltage and/or capacity requirements for portable products such as two-way radios and cellular phones, a string of individual cells (each usually contained in a metal can) is coupled serially or in parallel. The main reason for containment in the cell metal casing is the use of liquid electrolyte. This eliminates the risk of contamination of the portable product equipment from electrolyte leakage. Each cell's metal can is insulated in thin plastic heat shrink wrap to prevent shorting on contact with each other before insertion into the plastic housing. Each cell is then coupled typically using nickel or plated tabs, and then finally the cell pack is held within a hard plastic housing.
With the advent of solid electrolyte and solid conductive polymer materials, a new foundation for battery technology was laid. Conductive polymer materials are semiconductors in their natural state, but they can become conductive when doped (treated) with electron donor materials, i.e., alkali metals, or with electron acceptor materials, i.e., I.sub.2, AsF.sub.5, SO.sub.3, HSO.sub.3 F. These conductive polymers or plastics have a ring structure with delocalized .pi. bonding electrons which are relatively free to move about the polymer chain structure. Conductive polymers such as polyphenylene, polypyrrole, and polyaniline have been used as electrode materials. Polyaniline and its derivatives, in particular, have become of greater interest because of their higher stability in air and their ease of reproducibility. Some of these polymers can be made into thin flexible films which can be shaped or molded into convenient shapes for design compatibility. In addition to the conductive polymer electrodes, polymer electrolytes have been developed which can also be fabricated in thin plastic sheets. Some examples include polyethyleneoxide (PEO), polyphosphazene, polyether-substituted phosphazene (MEEP), and polypropyleneoxide. A list of conductive polymers under development can be found in Material Engineering journal, August 1985 issue, page 35, Table 1.
These solid state batteries have the one drawback of low conductivity at normal or ambient operating temperatures. This limits the current rate capability that can be obtained from the system. However, conductivities in the order of 10.sup.-4 /.OMEGA.-cm at room temperatures have been attained in laboratories, which are sufficient for some battery applications such as computer memory back-up and memory cards. Solid polymers made into thin films can provide higher rate capabilities by increasing surface area significantly. Therefore, voltages and capacities are obtained which widen the range of applications for thin film solid state batteries. In addition to the solid state batteries mentioned, some type of supercapacitors can also be used as power sources. Recently, a paper presented by Dr. S. D. Bhakta of the Materials Research Laboratory of SRI International at the Electrochemical Society Meeting in Seattle, Wash., on Oct. 14-19, 1990 disclosed the use of polymeric materials in supercapacitors which could actually function as batteries. Therefore, the use of these emerging technologies to provide an integrated battery built into or as part of an equipment housing would result in smaller overall size, lighter overall weight, and lower fabrication cost of the portable equipment.